Reposted from the Cliff Mass Weather Blog
The weather community is excited about the forecast this weekend: Several of the global models predict an exceptionally unusual heat wave in the Pacific Northwest this weekend.
A heat wave so extreme that many places could experience their warmest temperature on record.
For every day. For every year. And the end of June, which is not the usual time for the big temperature records.
Waves are the reason for this heat wave
But there is something else … this forecast is very uncertain and what happens depends on the weather events over the western Pacific in the next few days.
Let me show you … the predictions are just insane.
First, here are the predicted temperatures for Sunday at 5:00 p.m., based on the award-winning European Center model. As high as 121F in the northern Central Valley of California, 113 and 115 in the Columbia Basin, and 104-105 in the Willamette Valley. Just around 90F near Seattle.
Portland is forecast to hit 109F on Monday afternoon and 121F is forecast near the Oregon-WA border.
The US model GFS also relies on exceptional warmth in the region. Below is a “Plume” forecast plot for Seattle with the high resolution forecast (blue line) and an ensemble of many lower resolution simulations.
On Monday, the high-resolution run in SEATTLE reached 111F (the highest record is 103F); it appears to be an outlier: most of the ensemble predictions (gray lines) are cooler, with their mean (black line) only approaching 90F.
The high resolution UW forecast system goes for 110F in Seattle and 120F in Portland (see below). Simply stunning, simply gorgeous
This is so crazy that I can’t believe it.
The origin of this potential heat wave is a huge high-pressure ridge with a high amplitude (see the forecast for Sunday 11:00 am). For our region, it would represent the strongest ridge in history.
What is the origin of this ridge? It appears to be forced by a tropical disturbance in the western Pacific that moves north until it interacts with the jet stream, resulting in a series of waves located downstream. It’s like deflecting a long rope and spreading all kinds of waves.
The forecast of the upper current (approx. 30,000 ft) over the Pacific shows the development of the ripple (see below). Think of the lines as the print and the shading that tell you how unusual the winds are. Wind barbs are also indicated.
On Tuesday morning, the jet stream is relatively straight over the western Pacific and a weak fault is moving north (indicated by an arrow).
On June 24th, the jet stream began to buckle, with big waves over the North Pacific.
And on the 26th, the wave intensified to form a huge ridge over our region
But here’s the thing. Such interactions between a tropical disturbance and the jet stream are very delicate. Slight changes in the amplitude of the perturbation and where it meets the jet stream can result in large changes in the generation and movement of the jet stream waves.
This creates a great deal of uncertainty. Everything should have calmed down by Wednesday and we should have more confidence in the forecast.
My own guess, from a variety of predictions, is that Seattle will avoid the worst going north, with temperatures not rising until the mid-1990s, but Portland, the Willamette Valley, and the Columbia Basin will see historic, extraordinary highs in temperatures.